It is generally accepted, that increasing oil prices and national countries' legislations that require the reduction of automotive carbon dioxide emissions, force tire and rubber producers to contribute to produce “fuel efficient,” and thus fuel or gas saving tires. One general approach to obtain fuel efficient tires is to produce tire formulations that have reduced hysteresis loss. A major source of hysteresis in vulcanized elastomeric polymers is believed to be attributed to free polymer chain ends, that is, the section of the elastomeric polymer chain between the last cross-link and the end of the polymer chain. This free end of the polymer does not participate in any efficient elastically recoverable process, and as a result, any energy transmitted to this section of the polymer is lost. This dissipated energy leads to a pronounced hysteresis under dynamic deformation. Another source of hysteresis in vulcanized elastomeric polymers is believed to be attributed to an insufficient distribution of filler particles in the vulcanized elastomeric polymer composition. The hysteresis loss of a cross-linked elastomeric polymer composition is related to its Tan δ, at 60° C., value (see ISO 4664-1:2005; Rubber, Vulcanized or thermoplastic; Determination of dynamic properties—part 1: General guidance). In general, vulcanized elastomeric polymer compositions having relatively small Tan δ values, at 60° C., are preferred as having lower hysteresis loss. In the final tire product, this translates to a lower rolling resistance and better fuel economy.
It is generally accepted, that a lower rolling resistance tire can be made on the expense of deteriorated wet grip properties. For example, if, in a random solution styrene-butadiene rubber (random SSBR), the polystyrene unit concentration is relatively reduced, with respect to the total polybutadiene unit concentration, and the 1,2-polydiene unit concentration is kept constant, both the tan delta at 60° C. and the tan delta at 0° C., are reduced, generally corresponding to improved rolling resistance and deteriorated wet grip performance of a tire. Similarly, if, in a random solution styrene-butadiene rubber (random SSBR), the 1,2-polybutadiene unit concentration is relatively reduced, with respect to the total polybutadiene unit concentration, and the polystyrene unit concentration is kept constant, both the tan delta at 60° C. and the tan delta at 0α C. are reduced, generally corresponding to improved rolling resistance and deteriorated wet grip performance of a tire. Accordingly, when assessing the rubber vulcanizate performance correctly, both the rolling resistance, related tan delta at 60° C., and the wet grip, related tan delta at 0° C., should be monitored.
One generally accepted approach to reducing hysteresis loss is to reduce the number of free chain ends of elastomeric polymers. Various techniques are described in the open literature including the use of “coupling agents,” such as tin tetrachloride, which may functionalize the polymer chain end, and react with components of an elastomeric composition, such as, for example, with a filler or with unsaturated portions of a polymer. Examples of such techniques include: U.S. Pat. Nos. 3,281,383; 3,244,664 and 3,692,874 (tetrachlorosilane); U.S. Pat. Nos. 3,978,103; 4,048,206; 4,474,908; 6,777,569 (blocked mercaptosilanes) and U.S. Pat. No. 3,078,254 (a multi-halogen-substituted hydrocarbon, such as 1,3,5-tri(bromo methyl)benzene); U.S. Pat. No. 4,616,069 (tin compound and organic amino or amine compound); and U.S. 2005/0124740.
The application of “coupling agents,” as reactant to living polymers, more often than not, leads to the formation of polymer blends comprising one fraction of linear or uncoupled polymers, and one or more fractions comprising more than two polymer arms at the coupling point. The reference “Synthesis of end-functionalized polymer by means of living anionic polymerization,” Journal of Macromolecular Chemistry and Physics, 197, (1996), 3135-3148, describes the synthesis of “polystyrene-containing” and “polyisoprene-containing” living polymers with hydroxy (—OH) and mercapto (—SH) functional end caps, obtained by reaction of the living polymers with haloalkanes containing silyl ether and silyl thioether functions. The tertiary-butyldimethylsilyl (TBDMS) group is preferred as a protecting group for the —OH and —SH functions in the termination reactions, because the corresponding silyl ethers and thioethers are found to be both, stable and compatible with anionic living polymers.
International Publication No. WO 2007/047943 describes the use of a silane-sulfide omega chain end modifier, represented by the formula (RO)x(R)ySi—R′—S—SiR3, wherein x is the number one, two or three, y is the number zero, one or two, the sum of x and y is three, R is alkyl, and R′ is aryl, alkylaryl or alkyl, to produce a chain end modified elastomeric polymer, used as component in a vulcanized elastomeric polymer composition, or in a tire tread.
More specifically, according to WO 2007/047943, a silane-sulfide compound is reacted with anionically-initiated, living polymers to produce “chain end modified” polymers, which are subsequently blended with fillers, vulcanizing agents, accelerators or oil extenders, to produce a vulcanized elastomeric polymer composition having low hysteresis loss. To further control polymer molecular weight and polymer properties, a coupling agent (or linking agent) can be used according to WO 2007/047943, as an optional component, in the process of the preparation of elastomeric polymers. The modifier is then added before, after, or during, the addition of a coupling agent, and preferably, the modification reaction is completed after the addition of the coupling agent. In some embodiments, more than a third of the polymer chain ends are reacted with a coupling agent prior to addition of the modifier.
U.S. Pat. No. 5,502,131 describes a method of preparing a polymer comprising polymerizing diolefin monomers and/or monovinylaromatic monomers in the presence of a polymerization initiator having the general Formula A or B:

wherein R′1 and R′2 are same or different and are alkyls, cycloalkyls or aralkyls; R′3 is a deprotonated allyl, 2-methallyl or xylyl; R′4 is a carbocyclic group; and R′5 is an alkyl substituent on a methylene group. The formation of polymers comprising polar groups in the alpha and omega chain end positions was not conclusively demonstrated in the experimental section of the U.S. Pat. No. 5,502,131. Tan delta at 0° C. values correlating with the tire wet grip performance was not reported at all. Furthermore, the impact of the presented alpha chain end modified polymers in silica compound vulcanizates was not demonstrated or stated in the patent application. In addition, no heteroatom is included in R′4 in Formula B, and aromatic substituents are excluded for R′1 and R′2 in Formula A.
German Democratic Republic (GDR) patent applications DD 237513 A1, DD 242232 A1 and DD 236321 A1 describe a procedure for the preparation of multi-functional 1,3-diene homo- and copolymers (e.g. of butadiene with isoprene, styrene or alpha-methylstryrene), based on polymerization initiators of the general formula:
wherein n is a number from 2 to 6, R, R′, R″ and R′″ are each independently selected (but depending on the specific patent application DD 237513 A1, DD 242232 A1 or DD 236321 A1 selected) from the group consisting of alkyl, cyclic alkyl, aryl, allyl, deprotonated allyl or R″″—(CH2—CH(Li)—CH2)—, wherein and R″″ is an alkyl group, a cycloalkyl group or an aryl group. The molecular weights of the polymers as described in DD 237513 A1, DD 242232 A1 and DD 236321 A1 are too low to be used for compound mixtures useful for the application in tires. International Publication No. WO 2009/077837 A1 refers to a butadiene-styrene copolymer, functionalized at both extremities of its chains, to the preparation of the copolymers, to compounds comprising mentioned copolymers, and to the use of the same. In particular the patent publication refers to two groups of polymers, Group 1 representing alpha omega modified linear random styrene butadiene rubber, and Group 2 representing linear, branched and/or radial copolymer structures, as depicted in Scheme 1 below.

In Scheme 1, F1 represents a terminal functionalization of polymeric chains, and can be groups of the type —OH, —COOH, —COX, where X is a halogen, —SH, —CSSH, —NCO, epoxy and amine, and the amine group more specifically defined as one of the following structures: —N(R1)2, —NR2R2, —NHR1, NH2, wherein the groups R1 and R2 may be alkyl groups, cycloalkyl groups, aralkyl groups or aryl groups.
In Scheme 1, F2 represents one of the extremities of the polymer chains, functionalized with silyl, silanol and siloxane groups defined as one of the following structures —SiH2(OH), —Si(R1)2(OH), —SiH(OH)2, —SiR1(OH)2, —Si(OH)3, —Si(OR1)3, —(SiR1R2O)x—R3, —Si(R3)3—m(X)m, wherein X is a halogen, R1 and R2 are alkoxy, alkyl, cycloalkyl, aralkyl or vinyl groups, and R3 is hydrogen, alkyl, aryl or an amine group containing siloxane group represented by the formula -A1-Si(A2-N((H)k(R1)2−k))y(OR1)z(R3)3−(y+z), where k is the number 0, 1 or 2, y is the number 1, 2 or 3, z is the number 0, 1 or 2, 0≦y+z≦3, and R1, R2, R3, A1 and A2 are groups containing exclusively hydrogen and carbon atoms.
In Scheme 1, C is a silicon or tin based coupling agent with a functionality greater than, or equal to, the number 2, and represented by structures wherein the silicon or tin atom of the coupling agent is linked to a halogen, an —OR group or to a group containing exclusively hydrogen and carbon atoms, stated R group also represents a hydrocarbon group.
The application claims a butadiene and styrene copolymer containing “Group 1” (linear structure) and “Group 2” (branched or radial structure) butadiene-styrene copolymers, and one or more fillers, the nature of the fillers not being defined. In the patent application, there was no indication about the performance of the described polymers in carbon black compound vulcanizates.
Two typical fillers, silica and carbon black are applied to the tire production. Standard formulations very often comprise both fillers, silica and carbon black, in different ratios. Therefore, it would be desirably to have a modified polymer which gives excellent rolling resistance, and grip characteristics, in both carbon black and silica compounds. In addition, it would be desirable to have improved heat build-up values for the modified polymer-filler vulcanizate. A decreased heat built-up value reduces the risk of depolymerization in the vulcanizate in thermal and mechanical stress situations.
Additional initiator compounds and/or modifier compounds are described in the following: U.S. Pat. Nos. 5,502,131, 6,025,450, 6,080,835, 6,046,288, 5,792,820, 5,916,961, 5,866,650, 5,959,048, 5,852,189, 5,912,343, 5,736,617, 5,786,441, 7,342,070, 6,229,036, International Publication No. WO 2007/047943, International Publication Nos. WO/2009/148932 and WO/2010/056694.
There is a need for modification methods and resulting modified polymers that can be used to further optimize dynamic silica and carbon black vulcanizate properties, including low hysteresis properties, corresponding to a high wet grip and to a low rolling resistance property in tires. In addition there is a need to further decrease the vulcanizate heat built up during thermal exposure and under mechanical stress. These needs have been met by the following invention.